Recovery control in mirrored disks

ABSTRACT

A method for operating a mirrored disk storage system, comprises a determiner component for determining a repair characteristic of a pair of mirrored disks, a comparator for comparing said repair characteristic of a first of said pair with said repair characteristic of a second of said pair, and a cancelling component for cancelling a repair action for said one of said first or said second of said pair that is not selected by said selector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of a U.S. National-Stage entry under35 U.S.C §371, Ser. No. 13/529,047 filed Jun. 21, 2012, which is acontinuation of a U.S. National-Stage entry under 35 U.S.C §371, Ser.No. 13/125,771 filed Apr. 22, 2011, which is based on InternationalApplication No. PCT/EP2009/063678, filed Oct. 19, 2009, which waspublished under PCT Article 21(2) and which claims priority to EuropeanPatent Application No. 08169556.1, filed Nov. 20, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technology for recovery control inmirrored disks, and in particular to a technology for improving recoverytime of mirrored disks when read stability is in doubt.

2. Description of the Related Art

In storage systems an array of independent storage devices can beconfigured to operate as a single virtual storage device using atechnology known as RAID (Redundant Array of Independent Disks—earlierknown as Redundant Array of Inexpensive Disks). A computer systemconfigured to operate with a RAID storage system is able to performinput and output (I/O) operations (such as read and write operations) onthe RAID storage system as if the RAID storage system were a singlestorage device. A RAID storage system includes an array of independentstorage devices and a RAID controller. The RAID controller provides avirtualised view of the array of independent storage devices—this meansthat the array of independent storage devices appear as a single virtualstorage device with a sequential list of storage elements. The storageelements are commonly known as blocks of storage, and the data storedwithin them are known as data blocks. I/O operations are qualified withreference to one or more blocks of storage in the virtual storagedevice. When an I/O operation is performed on the virtual storage devicethe RAID controller maps the I/O operation onto the array of independentstorage devices. In order to virtualise the array of storage devices andmap I/O operations the RAID controller may employ standard RAIDtechniques that are now well known in the art.

In a non-RAID computer system, if a disk drive fails, all or part of thestored customer data may be permanently lost (or possibly partially orfully recoverable but at some expense and effort). Although backup andarchiving devices and procedures may preserve all but the most recentlysaved data, there are certain applications in which the risk of any dataloss and the time required to restore data from a backup copy isunacceptable. Therefore, RAID (“redundant array of inexpensive disks”)storage subsystems are frequently used to provide improved dataintegrity and device fault tolerance.

Storage subsystems thus aim to provide continuous data availability anddata integrity. One solution that aims to increase availability isRAID-1 which is also commonly known as mirroring. Mirroring maintainstwo or more copies of the data and when one copy is unavailable then theother, or another, copy is used to allow I/O to continue thus improvingavailability over the case where only a single copy exists and isunavailable.

To maintain the mirror each write I/O must be performed to each copy. AnI/O failure could occur before all write I/Os to all copies havecompleted which could result in the mirror having different data ondifferent copies. In these situations it is important that the storagesystem maintains read stability (which is defined to mean that everyread I/O to the same area should return the same data if no interveningwrites have occurred) which means that the copies must be restored to astate where they have identical data.

RAID-1 mirroring solutions typically have methods to store metadata torecord writes in flight that can be used to replay write I/Os after asystem failure (such as a reset). After the writes in flight have beenreplayed read stability is restored.

More severe system failures can mean all ability to replay writes inflight has been lost. In these situations there is no way to determinewhat part of the mirror copy data is identical. In these circumstancesread stability can be restored by choosing any one mirror copy as asource and copying all its contents to the other target copies (commonlyreferred to as synchronizing the copies). This situation when the mirrorread stability is in doubt is different from the case where one copy hasthe correct data and another copy does not contain the same data(because it could not be written to for some reason), recovery from thissituation using synchronisation is provided by systems known in the art.

As soon as the source copy is available data availability can berestored as this copy has the correct data that can be read. When thesource and all target copies are available and after the synchronisationprocess has successfully completed, read stability across the mirrorcopies has been restored and therefore all mirror copies are usable andmirror redundancy has been restored.

However, until the source copy is available the mirror is unavailableand the synchronisation process to restore the mirror redundancy cannotbe started. This means existing solutions can take a considerable timeto recover the mirror availability and its redundancy.

It would thus be desirable to have an improved technological means forrecovery control in mirrored disks, and in particular to have atechnology for improving recovery time of mirrored disks when readstability is in doubt.

SUMMARY OF THE INVENTION

The present invention accordingly provides, in a first aspect, anapparatus for operating a mirrored disk storage system, comprising: adetector component operable to detect that a failure has left readstability in doubt; a determiner component for determining a repaircharacteristic of a pair of mirrored disks; a comparator for comparingsaid repair characteristic of a first of said pair with said repaircharacteristic of a second of said pair; and a selector for selectingone of said first or said second of said pair having a preferred repaircharacteristic to be used as a synchronization source disk.

Said preferred repair characteristic may comprise the characteristicthat no repair is required for said one of said first or said second ofsaid pair, or that a least repair time is required for said one of saidfirst or said second of said pair. The preferred repair characteristicmay comprises the characteristic that no repair is required for said oneof said first or said second of said pair and that said one of saidfirst or said second of said pair is a space-efficient copy.

The apparatus may further comprise a cancelling component for cancellinga repair action for said one of said first or said second of said pairthat is not selected by said selector.

The apparatus may further comprise a synchronizing component forsynchronizing said first and said second of said pair using said one ofsaid first or said second of said pair having a preferred repaircharacteristic as said synchronization source disk.

In a second aspect, there is provided a method for of operating amirrored disk storage system, comprising the steps of: detecting that afailure has left read stability in doubt; determining a repaircharacteristic of at least a pair of mirrored disks; comparing saidrepair characteristic of a first of said pair with said repaircharacteristic of a second of said pair; and selecting one of said firstor said second of said pair having a preferred repair characteristic tobe used as a synchronization source disk.

Preferred features of the method provide steps corresponding to theactions of the components of the apparatus according to the preferredfeatures of the first aspect.

In a third aspect, there is provided a computer program comprisingcomputer program code to, when loaded into a computer system andexecuted thereon, cause said computer system to perform all the steps ofa method according to the second aspect.

Preferred embodiments of the invention contemplate, in their broadestaspect, technological means for recovery control in mirrored disks, andin particular a technology for improving recovery time of mirrored diskswhen read stability is in doubt.

In a fourth aspect, there is provided a method for operating a mirroreddisk storage system, comprises a determiner component for determining arepair characteristic of a pair of mirrored disks, a comparator forcomparing said repair characteristic of a first of said pair with saidrepair characteristic of a second of said pair, and a cancellingcomponent for cancelling a repair action for said one of said first orsaid second of said pair that is not selected by said selector.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described,by way of example only, with reference to the accompanying figures, inwhich:

FIG. 1 shows in schematic form an apparatus or arrangement of componentsin an initial state in which an embodiment of the present invention maybe implemented;

FIG. 2 shows in schematic form an apparatus or arrangement of componentsin an error state in which an embodiment of the present invention may beimplemented;

FIG. 3 shows in flowchart form one method or one logic arrangement foroperating an apparatus or arrangement of components to perform recoverycontrol in mirrored disks according to a general embodiment of theinvention;

FIG. 4 shows in flowchart form one alternative method or one logicarrangement for operating an apparatus or arrangement of components toachieve a further refined technological means for recovery control inmirrored disks according to a further embodiment of the invention; and

FIG. 5 shows in simplified schematic form an apparatus or arrangement ofcomponents adapted to perform the steps of a method according to apreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The preferred embodiment of the present invention is broadly directed toa technological means for recovery control in mirrored disks, and inparticular to a technology for improving recovery time of mirrored diskswhen read stability is in doubt.

The present invention may be implemented in configurations where themirror copies can require their own repair and recovery operation torestore data access during system failures. This is normally associatedwith repairing corruption of metadata on disk that if not repaired (orrebuilt entirely) would cause data integrity problems. The repairprocess can require time before that copy's availability is restored.Examples include:

The mirror copy is a file system that requires a repair (such as an FSCKrebuild);

The mirror copy is a space efficient copy that requires repair; or

The mirror copy is an IBM® SAN Volume Controller (SVC) Vdisk and the SVCcluster requires Tier 3 recovery. (IBM is a Registered Trade Mark ofInternational Business Machines, Corp. in the USA, other countries, orboth)

Preferred embodiments of the present invention provide an apparatus, amethod and a computer program operable to select the mirror with thebest recovery time and so restore the mirror availability and redundancyin less time (i.e. with an improved recovery time) or, in analternative, with other preferred characteristics.

Turning to FIG. 1, there is shown in simplified schematic form anapparatus or arrangement of components in an initial state in which apreferred embodiment of the present invention may be implemented.

In FIG. 1, storage system 100 comprises a host 102, and a RAIDcontroller 104. RAID controller 104 is operatively coupled to RAIDstorage devices 106, 108 which contain blocks of storage 110 in whichdata A and data B are stored in mirror image form.

Turning to FIG. 2, there is shown in simplified schematic form anapparatus or arrangement of components in an error state in which apreferred embodiment of the present invention may be implemented.

In FIG. 2, storage system 100 comprises a host 102, and a RAIDcontroller 104. RAID controller 104 is operatively coupled to RAIDstorage devices 106,108 which contain blocks of storage 110 in whichdata A and data B are stored in mirror image form. However, a controllerfailure indicated by “X” on the RAID controller 104 means that in-flightwrites are interrupted and controller metadata has been lost, such thatRAID devices 106, 108 are no longer correctly mirroring data, and thusread stability will not exist on the system when RAID storage devices106, 108 are re-established as a mirrored pair.

It is not clear on reestablishment which image would provide the correctdata in response to a read command, as data C could, for example, appearon RAID storage device 106 as a result of having been correctly writtenon RAID storage device 106 but not mirrored on RAID storage device 108,or as a result of having been correctly mirrored but subsequentlydeleted from RAID storage device 108 where the matching delete has notsucceeded on RAID storage device 106.

As will be clear to one of ordinary skill in the art, the arrangementand location of devices, systems or modules shown in the figures isintended to be exemplary only, and many other arrangements areenvisaged, as for example, locating devices at intermediate andcommunicating nodes of a data processing or communications network. Suchalternative arrangements of devices, systems or modules are wellunderstood by those of ordinary skill in the art, and need not befurther described here. Further, it will be clear to one of ordinaryskill in the art that the system depicted in the figures has been muchsimplified in order to display the storage system of an embodiment ofthe present invention, and that “real-life” systems will in many casesbe of considerably greater complexity. The advantageous nature of theembodiments of the present invention will be clearly seen by one ofskill in the art to inhere in the more complex arrangements as in thesimple arrangement depicted herein.

The general method of operation for mirrors that have their readstability in doubt may be represented in pseudo-code as follows:

set repair action time / ranking to a max value for each mirror copy {if mirror copy requires repair action { if this copy's repair actiontime / ranking is lower than recorded time / ranking { record mirrorcopy as source copy record this copy's repair action time / ranking asthe recorded time / ranking } } else { record mirror copy as source copyexit for each mirror copy loop (i.e. always use first copy that does notrequire repair as source copy) } } for each mirror copy { if mirror copyrequires repair action AND it is not the selected source copy { cancel /abort the copy repair action } } perform synchronisation from sourcecopy to all target copies (by reading data from source copy and writingto target copies).

FIG. 3 shows in a corresponding flowchart form a very simplifiedrepresentation of one method or one logic arrangement for operating anapparatus or arrangement of components to achieve improved technologicalmeans for recovery control in mirrored disks according to a generalembodiment of the invention.

In FIG. 3, after START step 300, the recorded value for repair time orrank is set to a maximum value determined according to parameters of thesystem, which, as will be clear to one of ordinary skill in the art, mayvary. At step 304 the first copy is taken for processing. At test step306, it is determined whether the copy needs any repair action. If thedetermination is negative, the first copy is set as the source copy forprocessing at step 318 and the following steps. If the determination attest step 306 is positive, the copy repair time is compared at test step308 with the recorded value. If the determination at test step 308 ispositive, the copy is set as source copy and the recorded value is setto the repair time or rank value of this copy and control passes to teststep 312. If, the determination at test step 308 is negative, controlpasses directly to test step 312. At test step 312, a determination ismade whether all copies have been processed, and if the determination isnegative, the next copy is taken for processing at step 314, after whichthe process repeats from test step 306. If the determination at teststep 312 is positive, control passes to step 318. At step 318, the firstcopy is taken for processing and control passes to test step 320. Teststep 320 determines whether the copy needs a repair action and is notthe selected source. If the determination is positive, the copy repairaction for the copy is cancelled at step 322 and control passes to teststep 324. Test step 324 determines if all copies have been processed. Ifthe determination is negative, the next copy is taken for processing atstep 326 and control returns to test step 320 for the next copy. If thedetermination at test step 324 is positive, all copies have beenprocessed, a source copy has been selected and control passes to step328. At step 328, mirror synchronization is performed using the selectedsource copy when it and the target copy or copies come back online oncompletion of any scheduled repair actions. At step 330, the processfinishes.

The core idea of each of the embodiments is to use knowledge of themirror copy repair characteristics to decide which is the best copy touse as the source for the mirror synchronisation from a recovery timeviewpoint (or, in an alternative, with other preferred characteristics).The required information for each copy according to the basic embodimentis:

Does copy need a repair action before it will be available again?

What is the expected repair time of the repair action? This can be anexplicit time value in seconds; or can be a relative value based onknowledge of copy configuration repair times that can be ranked (withquickest repair time first).

By selecting the mirror copy that has the smallest repair time or rank,the recovery time to restore availability can be minimized to therecovery time of the source copy repair action (if no source copy repairaction is required, the recovery time becomes 0—that is, availability isrestored immediately).

In addition a mechanism can be used to cancel the mirror copy repairaction for copies that are not the source. This mechanism notifies thecopy storage engine that its data is no longer needed (as it will all berewritten by the mirror synchronisation) and so its repair action can becancelled. This means the time to synchronize the target copy(s) andrestore the mirror redundancy has been reduced by the time the targetcopy repair actions would have taken (as the synchronisation cannotstart before the repair actions complete and the copies became availableagain).

Embodiments of the present invention include:

For hybrid mirrors with both Space Efficient and Fully Allocated copies,select the source copy to preserve space efficiency—this is important ifthe Space Efficient system does not support zero detection, as using theFully Allocated copy as the source will lead to the Space Efficient copybecoming fully allocated. The concepts of Space Efficient and FullyAllocated copies are well known in the art and need not be furtherdescribed here.

Use mirror synchronisation as an alternative to the copy repairmechanism when a mirror is healthy—this helps to improve systemredundancy recovery time when a synchronisation from another copy wouldbe quicker than the copy repair.

Some advantages of the preferred and alternative embodiments of thepresent invention over the prior art techniques are:

When mirror read stability is in doubt and full synchronisation isneeded and mirror copies have different repair times—there is adecreased recovery time for mirror availability;

When mirror read stability is in doubt and full synchronisation is andmirror copies have different repair times—there is a decreased recoverytime for mirror redundancy;

When mirror read stability is in doubt and full synchronisation isneeded—it is made possible to maintain space efficiency for spaceefficient implementations that do not implement zero detection; and

When mirror copies are all identical and synchronisation would bequicker than a required copy repair action—there is a decreased recoverytime for mirror redundancy.

For an alternative embodiment to be implemented, the followinginformation is also required:

Is the copy Space Efficient?

In a most preferred embodiment, each mirror copy records information inrespect of each of the following questions:

Is the copy space efficient (and not fully allocated)?

Does the space efficient copy require repair?

The most preferred implementation of the present invention then ranksthe repair times of different copy configurations as follows:

0==fully allocated copies or space efficient copies that do not requirea repair action

1==space efficient copies that require a repair action

Rank 0 is selected over rank 1.

In a further refinement, a modification of the basics system and methoddescribed above can be used to maintain space efficiency for hybridmirrors where the space efficient implementation does not support zerodetection.

This further refined mirror read-stability-in-doubt source-copyselection method can be represented as:

for each mirror copy { if mirror copy is space efficient { if mirrorcopy requires repair action { if source copy is already selected { donothing (i.e. do not select space efficient copy that requires repairover a non space efficient copy that is fully allocated) } else { recordmirror copy as source copy } } else { record mirror copy as source copyexit for each mirror copy loop (i.e. always use space efficient copythat does not require repair as source copy) } } else { record mirrorcopy as source copy } } for each mirror copy { if mirror copy is spaceefficient AND requires repair action AND it is not the selected sourcecopy { cancel / abort the space efficient copy repair action } } performsynchronisation from source copy to all target copies (by reading datafrom source copy and writing to target copies).

FIG. 4 shows in corresponding flowchart form this alternative method orlogic arrangement for operating an apparatus or arrangement ofcomponents to achieve a further refined technological means for recoverycontrol in mirrored disks according to a further embodiment of theinvention.

In FIG. 4, following start step 400, at step 402 the first copy is takenfor processing. At test step 404 the copy is tested to determine if itis a space-efficient copy. If the determination is negative, the copy isset as the source copy and control passes to test step 408. If thedetermination is positive, control passes to test step 412. Test step412 determines whether this copy needs a repair action, and if thedetermination is negative, sets this copy as the source copy at step416, before proceeding to process step 418. If the determination at teststep 412 is positive, control passes to test step 414. Test step 414determines whether a source copy has already been selected. If no sourcecopy has been selected yet, control passes to step 406, where this copyis set as the source copy and control passes to test step 408. If teststep 414 determines that a source copy has already been selected,control passes directly to test step 408. Test step 408 tests whetherall copies have been processed, and if the determination is positive,passes control to process step 418. If the determination at test step408 is negative, control passes to step 410 at which the next copy istaken for processing and control passes to test step 404 to begin thenext iteration. When all iterations are completed, as determined by teststep 408 described above, control passes to process step 418. At step418, the first copy is taken for processing and control passes to teststep 420. Test step 420 determines whether the copy is space efficient,needs a repair action and is not the selected source. If thedetermination is positive, the copy repair action for the copy iscancelled at step 422 and control passes to test step 424. Test step 424determines if all copies have been processed. If the determination isnegative, the next copy is taken for processing at step 426 and controlreturns to test step 420 for the next copy. If the determination at teststep 424 is positive, all copies have been processed, a source copy hasthus been selected and control passes to step 428. At step 428, mirrorsynchronization is performed using the selected source copy when it andthe target copy or copies come back online on completion of anyscheduled repair actions. At step 430, the process finishes.

As will be clear to one of ordinary skill in the art, a hardwareimplementation of the present invention may be provided, as shown in asimplified schematic form in FIG. 5. In FIG. 5 is shown an apparatus 500adapted to operate a mirrored disk storage system 502, comprising adetector component 504 operable to detect that a failure has left readstability in doubt. Determiner component 506 is operable to determine arepair characteristic of at least a pair of mirrored disks (for example,in the simplest case, the repair time for each disk). Comparator 508 isoperable to compare the repair characteristic of the first of the pairof disks with the repair characteristic of the second of the pair ofdisks. Selector 510 is operable to select the one of the first or secondof the pair having a preferred repair characteristic (for example, theshortest repair time) to act as the source disk for the mirrorsynchronization after it has been repaired. Canceller 512 is operable tocancel any repair actions for the copy or copies that have not beenselected as source copy. Synchronizer 514 performs the mirrorsynchronization after the copy that was selected as the source copy hasundergone any required repair actions.

It will be clear to one of ordinary skill in the art that all or part ofthe method of the preferred embodiments of the present invention maysuitably and usefully be embodied in a logic apparatus, or a pluralityof logic apparatus, comprising logic elements arranged to perform thesteps of the method and that such logic elements may comprise hardwarecomponents, firmware components or a combination thereof.

It will be equally clear to one of skill in the art that all or part ofa logic arrangement according to the preferred embodiments of thepresent invention may suitably be embodied in a logic apparatuscomprising logic elements to perform the steps of the method, and thatsuch logic elements may comprise components such as logic gates in, forexample a programmable logic array or application-specific integratedcircuit. Such a logic arrangement may further be embodied in enablingelements for temporarily or permanently establishing logic structures insuch an array or circuit using, for example, a virtual hardwaredescriptor language, which may be stored and transmitted using fixed ortransmittable carrier media.

It will be appreciated that the method and arrangement described abovemay also suitably be carried out fully or partially in software runningon one or more processors (not shown in the figures), and that thesoftware may be provided in the form of one or more computer programelements carried on any suitable data-carrier (also not shown in thefigures) such as a magnetic or optical disk or the like. Channels forthe transmission of data may likewise comprise storage media of alldescriptions as well as signal-carrying media, such as wired or wirelesssignal-carrying media.

A method is generally conceived to be a self-consistent sequence ofsteps leading to a desired result. These steps require physicalmanipulations of physical quantities. Usually, though not necessarily,these quantities take the form of electrical or magnetic signals capableof being stored, transferred, combined, compared, and otherwisemanipulated. It is convenient at times, principally for reasons ofcommon usage, to refer to these signals as bits, values, parameters,items, elements, objects, symbols, characters, terms, numbers, or thelike. It should be noted, however, that all of these terms and similarterms are to be associated with the appropriate physical quantities andare merely convenient labels applied to these quantities.

The present invention may further suitably be embodied as a computerprogram product for use with a computer system. Such an implementationmay comprise a series of computer-readable instructions either fixed ona tangible medium, such as a computer readable medium, for example,diskette, CD-ROM, ROM, or hard disk, or transmittable to a computersystem, via a modem or other interface device, over either a tangiblemedium, including but not limited to optical or analogue communicationslines, or intangibly using wireless techniques, including but notlimited to microwave, infrared or other transmission techniques. Theseries of computer readable instructions embodies all or part of thefunctionality previously described herein.

Those skilled in the art will appreciate that such computer readableinstructions can be written in a number of programming languages for usewith many computer architectures or operating systems. Further, suchinstructions may be stored using any memory technology, present orfuture, including but not limited to, semiconductor, magnetic, oroptical, or transmitted using any communications technology, present orfuture, including but not limited to optical, infrared, or microwave. Itis contemplated that such a computer program product may be distributedas a removable medium with accompanying printed or electronicdocumentation, for example, shrink-wrapped software, pre-loaded with acomputer system, for example, on a system ROM or fixed disk, ordistributed from a server or electronic bulletin board over a network,for example, the Internet or World Wide Web.

In one alternative, the preferred embodiment of the present inventionmay be realized in the form of a computer implemented method ofdeploying a service comprising steps of deploying computer program codeoperable to, when deployed into a computer infrastructure and executedthereon, cause said computer system to perform all the steps of themethod.

In a further alternative, the preferred embodiment of the presentinvention may be realized in the form of a data carrier havingfunctional data thereon, said functional data comprising functionalcomputer data structures to, when loaded into a computer system andoperated upon thereby, enable said computer system to perform all thesteps of the method.

It will be clear to one skilled in the art that many improvements andmodifications can be made to the foregoing exemplary embodiment withoutdeparting from the scope of the present invention.

1. A method of operating a mirrored disk storage system, comprising thesteps of: determining a repair characteristic of at least a pair ofmirrored disks; comparing said repair characteristic of a first of saidpair with said repair characteristic of a second of said pair; andcancelling a repair action for said one of said first or said second ofsaid pair that is not selected by said selector.
 2. The method of claim1, further including selecting one of said first or said second of saidpair having a preferred repair characteristic to be used as asynchronization source disk, wherein said preferred repaircharacteristic comprises a characteristic that no repair is required forsaid one of said first or said second of said pair.
 3. The method ofclaim 2, wherein said preferred repair characteristic comprises acharacteristic that a least repair time is required for said one of saidfirst or said second of said pair.
 4. The method of claim 3, whereinsaid preferred repair characteristic comprises a characteristic that norepair is required for said one of said first or said second of saidpair and that said one of said first or said second of said pair is aspace-efficient copy.
 5. The method of claim 1, further includingdetecting that a failure has left read stability in doubt.
 6. The methodof claim 1, further comprising a step of synchronizing said first andsaid second of said pair using said one of said first or said second ofsaid pair having a preferred repair characteristic as saidsynchronization source disk.
 7. An apparatus for operating a mirroreddisk storage system, comprising: a determiner component for determininga repair characteristic of a pair of mirrored disks; a comparator forcomparing said repair characteristic of a first of said pair with saidrepair characteristic of a second of said pair; and a cancellingcomponent for cancelling a repair action for said one of said first orsaid second of said pair that is not selected by said selector.
 8. Anapparatus as claimed in claim 7, further including a detector componentoperable to detect that a failure has left read stability in doubt and aselector for selecting one of said first or said second of said pairhaving a preferred repair characteristic to be used as a synchronizationsource disk, wherein said preferred repair characteristic comprises thecharacteristic that no repair is required for said one of said first orsaid second of said pair.
 9. An apparatus as claimed in claim 8, whereinsaid preferred repair characteristic comprises a characteristic that aleast repair time is required for said one of said first or said secondof said pair.
 10. An apparatus as claimed in claim 9, wherein saidpreferred repair characteristic comprises a characteristic that norepair is required for said one of said first or said second of saidpair and that said one of said first or said second of said pair is aspace-efficient copy.
 11. An apparatus as claimed in claim 7, furthercomprising a synchronizing component for synchronizing said first andsaid second of said pair using said one of said first or said second ofsaid pair having a preferred repair characteristic as saidsynchronization source disk.
 12. A physical computer non-transitorystorage medium comprising a computer program product method foroperating a mirrored disk storage system, the physical computernon-transitory storage medium comprising: computer code for determininga repair characteristic of at least a pair of mirrored disks; computercode for comparing said repair characteristic of a first of said pairwith said repair characteristic of a second of said pair; and computercode for cancelling a repair action for said one of said first or saidsecond of said pair that is not selected by said selector.
 13. Thephysical computer non-transitory storage medium of claim 12, furtherincluding computer code for detecting that a failure has left readstability in doubt and computer code for selecting one of said first orsaid second of said pair having a preferred repair characteristic to beused as a synchronization source disk, wherein said preferred repaircharacteristic comprises a characteristic that no repair is required forsaid one of said first or said second of said pair.
 14. The physicalcomputer non-transitory storage medium of claim 13, wherein saidpreferred repair characteristic comprises a characteristic that a leastrepair time is required for said one of said first or said second ofsaid pair.
 15. The physical computer non-transitory storage medium ofclaim 14, wherein said preferred repair characteristic comprises acharacteristic that no repair is required for said one of said first orsaid second of said pair and that said one of said first or said secondof said pair is a space-efficient copy.
 16. The physical computernon-transitory storage medium of claim 12, further comprising computercode for synchronizing said first and said second of said pair usingsaid one of said first or said second of said pair having a preferredrepair characteristic as said synchronization source disk.